Abstract Compound-specific carbon and hydrogen isotope ratios are routinely measured on extractable organic matter to decipher biogeochemical processes and events in Earth history. To deliver accurate interpretations, it is paramount that isotopic values are derived from indigenous compounds and are not the result of contamination. However, distinguishing between compounds from these different provenances can be difficult, especially if a degree of mixing occurred. In this study, we assess the impact of hydrocarbon contamination on the carbon and hydrogen isotopic composition of n-alkanes from ∼820 Ma Precambrian evaporitic drill core samples through exterior/interior (E/I) rock extraction experiments. In these experiments, exterior and interior portions of the same rock samples were separately crushed to powder, extracted and processed. Compound-specific isotope values of n-alkanes from the different rock portions were subsequently measured and compared. In most cases, n-alkanes from exterior rock portions had consistently more depleted δ13C and δ2H values than their interior counterparts with an E/I isotopic offset averaging 0.1–6.9‰ for δ13C and 2–33‰ for δ2H. These diverging isotope patterns tend to correspond to E/I concentration differences of n-alkanes and are the result of contaminants overprinting on indigenous isotopic signals. Through the application of E/I experiments, the degree of isotopic overprinting can be investigated and mixed indigenous/contaminant signals identified.